config: added backward compatibilty

Added backward compatibility supporting upgrade from
3.1 version which used the keyword rhevm instead of engine.
The old config files will be backed up during the upgrade.

cherry picked with modifications from
5872ba1398647e0d5f2b3c471d198c54b8ef8d9c

Change-Id: I781493d9a224e412edbf1816579718de39ce0f54
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>